Solar photovoltaic devices, i.e., solar cells, are devices capable of converting solar radiation into usable electrical energy. The energy conversion occurs as the result of what is known as the photovoltaic-effect which occurs in a cell composed of a p-type semiconductor layer adjacent to an n-type semiconductor layer, hereafter referred to as a p-n junction cell. Solar radiation impinging on a solar cell and absorbed by an active region of semiconductor material generates electricity.
Multi-junction solar cells may be more efficient than single-junction solar cells if properly designed. One such design is described in U.S. Pat. No. 5,223,043 issued to Olson et al. Important considerations to achieve high efficiency energy conversion include the following: a) high quality crystalline layers; b) appropriate choice of junction band-gaps based on the impinging solar spectrum; c) tunnel junction interconnects between p-n junctions; d) appropriate choice of layer thicknesses to achieve a current-matched structure; and e) passivating layers, such as back-surface-field layers or window layers, to reduce losses. In the past, high-efficiency III-V semiconductor multi-junction solar cells have been grown on GaAs, InP, and Ge substrates, but silicon substrates have been found advantageous for cost and mechanical robustness reasons.
Alloys containing the atoms (AlGaIn)(PAsSb) are examples of III-V semiconductors, so named because their constituent elements come from the columns IIIb and Vb of the periodic table. In the past, solar cells consisting of high-quality, single-crystal layers of (AlGaIn)(PAsSb) semiconductor alloys with a large range of optical properties have been grown on GaAs, InP, and Ge substrates because these alloys can be fabricated with compositions such that the crystal lattice parameter and crystal symmetry match that of the underlying substrate. This “lattice-matching” condition results in epitaxial layers with minimal strain, few defects and thus superior electrical properties. Unfortunately, the set of semiconductors alloys (AlGaIn)(PAsSb) cannot be lattice-matched to silicon for any composition.
In the past, many investigators have attempted to grow III-v solar cells on single-crystal silicon substrates. Blakeslee et al. (U.S. Pat. No. 4,278,474), Umeno et al. (U.S. Pat. No. 4,963,508), and Ringel et al. (U.S. Pat. No. 5,571,339) have all disclosed lattice-mismatched III-V solar cell devices grown on silicon substrates using strain-relieving buffer layers. But because these III-V solar cell designs are not lattice-matched to the underlying silicon, problems with high defect densities in the III-V semiconductor layers have prevented such solar cell designs from achieving efficiencies as high as those on GaAs or Ge substrates.
The addition of small amounts of boron (B) and/or nitrogen (N) to the more standard III-V alloys does allow for compositions lattice-matched to silicon to be reached. For example, GaNxP1-x-yAsy is lattice-matched to silicon for 0.022<x<0.194 and y=4.6x−0.09. The ability to fabricate these semiconductor alloys with nitrogen or boron concentrations greater than about 0.1% has only recently been discovered and the achievable compositions and their properties are under current investigation.
In the recent past, GaNxP1-x, GaInyNxP1-x, and GaNxP1-x-yAsy have been grown on Gap and Si substrates for light emitting applications. GaNxP1-x has also been shown to have a direct (or direct-like) band gap. BxGa1-x-yInyAs has been grown on GaAs, but would require considerably greater concentrations of boron to be lattice-matched to silicon. BxGa1-xP has not been attempted but would have a much better chance to be lattice-matched with silicon than BxGa1-x-yInyAs. All of these III-V semiconductors have typically been grown using metal-organic vapor phase epitaxy (MOVPE), molecular beam epitaxy (MBE), and similar techniques.